Controllers for electric motors provide electrical energy to the motor for proper operation. Typically, the controller applies a voltage to the motor at a prescribed frequency. The voltage and the frequency are chosen to optimize the speed and torque output of the motor.
It has been shown that with an AC induction motor, the torque output of the motor can be improved if an over-voltage condition is applied. During this condition, the motor is operated in a partial saturation condition due to the magnetization current at low loads. However, the efficiency of the motor is decreased at lower speeds when operated in the over-voltage condition.
Referring to FIG. 1, a graph showing the efficiency of an AC induction motor operating in various conditions is shown. In FIG. 1, line 10 illustrates the efficiency of a standard AC induction motor operating in an over-voltage condition. As can be seen, the efficiency of the motor is low (i.e., approx. 50–75%) at low load conditions and the efficiency increases as the load increases. An AC induction motor operating normally is shown as line 12 and as can be seen, the efficiency of the motor is greatest at low load conditions and decreases as the load on the motor increases.
Currently, motor controllers can use vector controls for operating the motor at maximum efficiency. Vector controls are complex mathematical formulas which model the operation of the motor and use real-time monitoring of the motor. Specifically, the vector controls are a closed-loop feedback system that control the phase relationships between the input voltages. In order for the vector control to be effective, very sensitive measurements of the operating parameters of the motor are needed. In this respect, vector controls require very sensitive and expensive sensors to measure the operation of the motor.
Accordingly, there is a need for a motor controller to operate an electric motor in an efficient manner at different load conditions without the use of sensitive or complex control techniques.